It is very necessary to monitor the state of blood oxygen for patients in the process of medical operation and recovery. Generally, this is performed by monitoring the blood oxygen saturation (namely the oxygen content of arterial blood). In a human body, the arterial blood pulsates in the ends of tissues as a result of the pulse wave, and the oxyhemoglobin (HbO2) and the reduced hemoglobin (Hb) cause the media to be measured (such as fingers or toes, etc.) to have different transmittivities of red light and infrared light. Nowadays, domestic or foreign pulse oximeters operate by utilizing the above principle, that is: irradiating red light and infrared light of a certain intensity to the media to be measured, detecting the transmitted light intensities of the two lights, and then calculating the blood oxygen saturation based on the ratio of the density variations of the red light and the infrared light after the two lights passing through the media to be measured such as fingers.
Based on the measuring principle described above, a device for non-traumatic measurement of blood oxygen saturation basically comprises a blood oxygen sensor and a signal processing unit. The key component of the blood oxygen sensor is a sensor including a light-emitting diode (LED) and a photosensor. At one side of the sensor, the LED can provide lights of two or more wavelengths; and at the other side of the sensor, the photosensor can convert the light signals passing through the measured media between the LED and the photosensor and containing the information of blood oxygen saturation into electrical signals which are transmitted to the signal processing unit to be digitized for calculating the blood oxygen saturation.
FIG. 1 shows the main structure of the blood oxygen sensor. As shown in FIG. 1, a blood oxygen sensor 1 comprises a sensor head 11, a signal transmission cable 12 and a connector 13. The sensor head 11 comprises a photodiode 111, a first LED 114 (emitting red lights) and a second LED 115 (emitting infrared lights), the first LED 114 and the second LED 115 being connected in inverse-parallel with each other. The signal transmission cable 12 comprises an external shielding layer 126, an internal shielding layer 121, a first core 122 and a second core 123, the first core 122 and the second core 123 being connected to a cathode pin 112 and an anode pin 113 of the photodiode 111, respectively, for transmitting the current modulated by the photodiode 111. Moreover, the first core 122 and the second core 123 are enwrapped by the internal shielding layer 121. The signal transmission cable 12 further comprises a third core 124 and a fourth core 125 in parallel with the internal shielding layer 121, the third core 124 and the fourth core 125 being connected to both pins of the first LED 114, respectively, for transmitting the current which alternately drives the first LED 114 and the second LED 115. The sensor head 11 is connected to a signal processing unit 2 (for example, a blood oxygen module circuit) via the signal transmission cable 12 and the connector 13 in turn. Thus, the lights transmitting through the tissue can be received by the blood oxygen sensor and converted into electrical signals, and then supplied to subsequent devices for further processing.
The normal operation of the oximeter for measuring the blood oxygen saturation is based on the reliable and fault-free blood oxygen sensor. As the sensor head 11, the signal transmission cable 12 and the connector 13 are movable components, they tend to be damaged in use. For example, the first core 122 and the second core 123 may be in short-circuit connection with the internal shielding layer 121, or the third core 124 may be in short-circuit connection with the fourth core 125, or the first LED 114 or the second LED 115 may be short-circuited. All these faults will cause the oximeter to operate abnormally and output incorrect data. Therefore, it is necessary to monitor the fault of the blood oxygen sensor 1 and output prompt messages in the signal processing unit 2.
Generally, the intensities of the current signals coming from the blood oxygen sensor vary depending on the intensities of the pulse wave signals of the human body. But if any of the above-mentioned faults occurs in the sensor, the photodiode cannot output the light-modulated current signals whose intensities vary depending on the intensities of the pulse wave signals of the human body. The signal processing unit 2 existing in prior arts can judge whether or not any fault occurs in the sensor on the basis of the above feature.
However, there are the following limitations and disadvantages in the prior arts described above. Only when the measured part of the human body is positioned in the sensing range of the sensor can the signal processing unit judge whether or not any fault occurs in the sensor on the basis of the characteristic of the pulse wave of the human body. Furthermore, if the photodiode outputs a signal whose characteristic is similar to that of the pulse wave as a result of other factors such as ambient lights, a wrong determination will be made by adopting the above-mentioned method. Thus, it's not sure whether the fault can be found in time once the fault occurs in the blood oxygen sensor, and this will affect the monitoring of the blood oxygen of patients.